Typical electric power cables generally comprise one or more conductors in a cable core that is surrounded by several layers of polymeric materials including a first semi-conducting shield layer (conductor or strand shield), an insulating layer, a second semi-conducting shield layer (insulation shield), a metallic tape or wire shield, and a protective jacket. The outer semi-conducting shield can be either bonded to the insulation or strippable, with most applications using strippable shields. Additional layers within this construction, such as moisture impervious materials, are often incorporated.
Many cable sheaths, e.g., an insulation sheath, a protective jacket, etc., are prepared from high pressure polyolefins. e.g., high pressure low density polyethylene (HPLDPE). The manufacture of these polyolefins requires, as their name implies, high operating pressures, e.g., operating pressures of 70 to 350 megapascals (MPa, or about 10,000 to 50,000 psi) are typical with operating pressures of 240 to 310 MPa (about 35,000 to 45,000 psi) preferred. To achieve these high pressures, one or more hypercompressors are employed, and the operation of this equipment requires the use of lubricants. Unfortunately, given the high operating pressures and the nature of commercially available hypercompressor seals, lubricant inevitably leaks into the reactor, albeit at very low levels (e.g., parts per million) to mix with and become part of the reaction mass, e.g., ethylene, comonomer, solvent, catalyst, etc., and ultimately the reaction product, i.e., the high pressure polyolefin.
Traditionally, mineral oil has been used as a hypercompressor lubricant, but it is associated with substantial maintenance time for the hypercompressors. Polyhydroxy-functional polyalkylene oxide co-polyols, such as UCON™ PE-320, which is available from The Dow Chemical Company, is another group of hypercompressor lubricants. While these lubricants are generally better than mineral oil in the context of hypercompressor maintenance, their presence in the high pressure polyolefin product can have an adverse affect on the use of the polyolefin product both in processes in which the polyolefin is eventually crosslinked, and in the performance of the polyolefin in its intended use, even if the lubricant is present only in parts per million amounts.
Due to the presence of multiple hydroxyl groups and the hydrophilic ethylene oxide groups, these lubricants are quite hydrophilic. This can result in increased water uptake by the polymer and this, in turn, can adversely affect the properties of the article made from the polymer. For example, the presence of unwanted water and the polyhydroxyl functionality on the lubricant can increase the electrical losses of an insulation sheath made from the polyolefin when the sheath is exposed to the high electrical stress conditions of a medium or high voltage power cable. Increased electrical losses will, in turn, shorten the useful life of the power cable. Moreover, the presence of unwanted water and the polyhydroxyl functionality on the lubricant can result in scorch, i.e., pre-mature crosslinking of the polyolefin, during the process of fabricating the insulation sheath.
Accordingly, the polymer fabrication industry, particularly the wire and cable industry, has a continuing interest in compositions and methods for preparing cable sheaths comprising a high pressure polyolefin that will reduce electrical losses in medium and high voltage power cables.